Place zucchini in a saucepan, and fill with enough water to cover. Bring to a boil, and cook until tender, about 5 minutes. Drain, and transfer to a food processor or blender. Process until smooth. Add the garlic, sugar, and oregano, and process until blended.
Transfer the pureed mixture to a serving bowl, and stir in the mayonnaise. Chill for at least 1 hour before serving.